The field of the invention relates to air/fuel ratio control for motor vehicles having a fuel vapor recovery system coupled between the fuel supply system and the air/fuel intake of an internal combustion engine.
Feedback control systems responsive to an exhaust gas oxygen sensor are commonly employed to maintain the engine's air/fuel ratio at a desired value. Typically, a two-state exhaust gas oxygen sensor is utilized which provides an output signal having either a high voltage state or a low voltage state when the engine is operating on the rich side or lean side, respectively, of the desired air/fuel ratio. This output signal is usually integrated to provide a measurement of average air/fuel ratio which is then used as a feedback variable for regulating fuel delivered to the engine.
It is also known to generate a second feedback variable for correcting engine conditions which may cause permanent or long term air/fuel ratio offsets. For example, a fuel injector having an oversized orifice will provide a continuous air/fuel offset in the rich direction. Rather than have the first feedback variable continuously correcting for such offsets, a second feedback variable is generated in response to the overall offset of the first feedback variable. Delivered fuel is then corrected in response to both feedback variables.
Air/fuel ratio control has been complicated by the addition of fuel vapor recovery systems to motor vehicles. To reduce emissions of gasoline vapors into the atmosphere, as required by government emission standards, fuel vapor recovery systems are commonly utilized. These systems store excess fuel vapors emitted from the fuel tank in a canister having activated charcoal or other hydrocarbon absorbing material. To replenish the canister storage capacity, air is periodically purged through the canister, absorbing stored hydrocarbons, and the mixture of vapors and purged air inducted into the engine. Concurrently, vapors are inducted directly from the fuel tank into the engine.
A prior approach to air/fuel feedback control for an engine which is coupled to a fuel vapor recovery system is disclosed in U.S. Pat. No. 4,467,769 issued to Matsumura. Delivered fuel is adjusted in accordance with two feedback variables. The first feedback variable, an integration correction amount, is derived from the output signal of an exhaust gas oxygen sensor. A learning correction amount is then generated as a second feedback variable from the integration correction amount during steady-state engine operations. This learning correction amount is utilized to compensate for long-term or permanent air/fuel ratio offsets caused by engine operation. When steady-state engine operation is indicated by comparing measurements of inducted airflow and other engine operating parameters, a learning correction amount is provided and stored as a function of mass airflow. Stated another way, during steady-state engine operation, a look-up table is generated of inducted airflow versus learning correction values. In addition, when the engine is detected as being in steady-state operation, the fuel vapor recovery system is disabled to facilitate the learning operation.
The inventors herein have recognized several disadvantages of the above approaches. For example, disabling fuel vapor recovery whenever the engine is at steady-state operation may result in excessive emission of fuel vapors into the atmosphere and over-pressurization of the fuel system. This disadvantage may be particularly troublesome during highway cruising when the engine is at steady-state operation for a long period of time. In addition, tighter government regulations governing hydrocarbon emissions in the near future will cause such approaches to become particularly troublesome.